There is a need to measure the cetane numbers, octane numbers and like parameters, of various kinds of fuels, such as middle distillate fuels and gasolines. The cetane numbers relate to the ignition delay characteristics, the low temperature starting ability, and the exhaust emission properties of the fuel for a given engine configuration. The octane numbers relate to fuel properties that enable avoidance of sudden explosive combustion conditions known as knock for a given engine configuration.
Prior techniques involve carrying out combustion tests in a combustion chamber. The fuel must be injected into the chamber under carefully controlled conditions. Prior art methods involve the use of a barrel and plunger, but are not sufficiently well controlled, particularly with regard to the quantity of fuel, the rate of injection, and the pressure at which the fuel is injected into the combustion chamber. All these parameters must be very precisely controlled in order to obtain accurate repeatable measurements. Furthermore, prior art techniques do not lend themselves to continual on-line monitoring.
Current Applicant sought to alleviate these problems with an air-assisted fuel injection system which is described in U.S. Pat. No. 5,906,190, the contents of which are incorporated by reference herein. This system relies on an in-line piston which acts directly on the plunger to ensure positive contact.
However, this system is still subject to some inconsistency and variation in the quantity of fuel injected into the combustion chamber. The present inventors conducted numerous, long-term experiments trying to determine the cause of these variations. The inventors experimented with temperature and air control settings, the pump settings to the injection settings and the reservoir, none of which resulted in any reduction in these variations.
Typical existing injection systems have loose tolerances and good repeatability, particularly of the injection profile, has been very difficult if not impossible to achieve. A multitude of problems arise when trying to inject accurately small quantities of fuel.